This invention relates to reconcentration of desiccants used in dehydration of gases. More particularly, the invention relates to reconcentration of such desiccants as glycols used in the dehydration of fuel gases such as natural gas, while controlling or preventing discharge of pollutants such as VOCs (volatile organic compounds), H2S (hydrogen sulfide) gas and other gaseous (including vaporous) pollutants into the atmosphere.
Heretofore, drying fuel gases such as NG with liquid desiccants such as TEG was widely acknowledged as both acceptable and effective. Many commercial units have been constructed to practice this method.
Water absorbed from the gas by the desiccant in such installations could be removed from the xe2x80x9cwetxe2x80x9d desiccant in reconcentrating equipment. Thus freed of most of the water, the resulting xe2x80x9cleanxe2x80x9d desiccant could be recycled for use in drying additional gas.
A common type of reconcentrating device included a reboiler and an associated column having a reflux section. These components cooperated to vaporize most of the water and a relatively small proportion of the desiccant, ejecting most of the water as vapor while returning most of the desiccant to the liquid phase.
An extensive patent literature has developed concerning this mode of processing, including for example U.S. Pat. No. 3,105,748 (Stahl), U.S. Pat. No. 3,347,019 (Barnhart), U.S. Pat. No. 3,370,636 (Francis, Jr., et al), U.S. Pat. No. 3,450,603 (Meyers et al), U.S. Pat. No. 3,451,897 (Welch), 3,736,725 (Alleman), U.S. Pat. No. 3,824,177 (Honerkamp et al), U.S. Pat. No. 3,841,382 (Gravis III et al), U.S. Pat. No. 4,010,065 (Alleman), U.S. Pat. No. 4,026,681 (Roskelley), U.S. Pat. No. 4,070,231 (Alleman), U.S. Pat. No. 4,182,659 (Anwer et al), U.S. Pat. No. 4,280,867 (Hodgson), U.S. Pat. No. 4,322,265 (Wood), U.S. Pat. No. 4,460,383 (Valerius) and U.S. Pat. No. 5,084,074 (Beer et al).
TEG and other popular desiccants are subject to thermal decomposition, which occurs to a greater or lesser extent depending upon the time/temperature history of the desiccant. Decomposition, if excessive, unduly increases operating costs by requiring removal of undue amounts of tars or chars from the system while also creating a corresponding need to purchase makeup desiccant. As a consequence, reboiler operating temperature has been limited to about 400xc2x0 F. or less when reconcentrating TEG.
Those practicing in this art have long known that, other factors remaining equal, desiccant decomposition can be reduced through operating at lower desiccant temperatures while maintaining the total vapor pressure in the gas spaces of the reboiler and reflux sections of the reconcentrator at relatively low levels. Thus, it was quite common to operate such equipment at sub-atmospheric or at substantially atmospheric pressure. For example, see U.S. Pat. No. 4,322,265 to Wood.
In U.S. Pat. No. 5,084,074, to exclude entry of atmospheric oxygen into their system, Beer et al suggested using a slightly positive pressure in an accumulator unit indirectly connected with their reboiler. Other workers in the art have recommended beginning the water removal at super-atmospheric pressures, but completed the removal of water in a chamber at sub-atmospheric pressure without a heating device. See for example U.S. Pat. No. 3,824,177 to Honerkamp et al and U.S. Pat. No.4,182,659 to Anwer et al.
Customarily, the water ejected from the desiccant during its reconcentration was released into the atmosphere. Unfortunately, water vapor that was so released carried with it a variety of pollutants.
Because these pollutants are typically present in natural gas and other fuels, and because typical desiccants such as TEG and other glycols have an affinity for such pollutants, the pollutants were present in the used desiccant when the latter was processed in the above-described reboilers and their reflux sections. The character of the pollutants and of the conditions within the reconcentrating units were such that substantial amounts of these pollutants were released into the atmosphere in admixture with the water vapor.
Examples of such pollutants include the above-mentioned VOCs and H2S. The VOCs are composed primarily of aromatic compounds, such as benzene, toluene an xylene, known carcinogens. (H2S) is a poisonous gas. It is unfortunate that TEG, being one of the most popular desiccants for use in such processes, has a tremendous affinity for aromatics and ejects large quantities of same along with the water vapor. TEG also absorbs enough (H2S) to cause similar difficulties.
In response to these difficulties, persons skilled in the art began the practice of cooling and condensing the water and pollutants released from the reflux sections of reconcentrating units. Condensed VOCs were generally pumped back into the system.
Depending on the amounts of VOCs present and the amount of cooling applied to the vapors, such cooling was not always able to recover sufficient VOCs to meet environmental standards, but, when used, increased the capital and/or operating costs of the reconcentrating unit. Moreover, such measures provided only marginal control of H2S at best, and in many cases the fuel and system characteristics were such that H2S could not be adequately controlled to meet environmental requirements.
There was also some use of vapor compressors to assist in the collection of H2S gas, non-condensed VOCs, and other non-condensed gases after discharge at atmospheric pressure or near atmospheric pressure from the reflux section. However, due to the relatively high investment and operating costs along with additional problems associated with compressors having a suction pressure at or near atmospheric pressure, this method has not proven to be popular. This method typically required investment in a compressor dedicated to the recovery of the vapors, which would otherwise not be required in the process. In addition, small deviations in the control system controlling the vapor compressor suction pressure at or near atmospheric pressure creates upsets in the dehydration process and when the deviation allows the suction pressure to drop below atmospheric pressure, air can be introduced in the system. Air, when introduced into the dehydration system causes severe corrosion and degradation of the desiccant and, in addition, creates the possible hazard of an explosive mixture being developed in the dehydration and compression system.
A rather widely used apparatus for removing the last traces of water from partially reconcentrated desiccant was a gas stripper, as described in U.S. Pat. No. 3,105,748 to Stahl. The stripper received the partially reconcentrated liquid desiccant from the reboiler and subjected it, under conditions promotive of mass transfer, to countercurrent contact with a stripping gas, usually NG, which had an affinity for water and thus absorbed water from the desiccant. The xe2x80x9cwetxe2x80x9d NG was then returned to the reboiler and was eventually discharged therefrom in admixture with water vapor, VOCs, H2S and possibly other gases. However, from time to time, escalating NG prices have exerted economic pressure on the use of stripping, and have supported a trend in the art toward finding ways to minimize or avoid the use of stripping gas. These included, among others, vacuum distillation, atmospheric distillation in the 425-430xc2x0 F. temperature range, and azeotropic distillation.
As a result of these circumstances, a need has arisen for environmentally acceptable gas drying methods which are practical and economical. The present invention was developed to meet this need.
The invention has a number of novel and non-obvious aspects, enumerated below, which may be employed singly or in any combination with one another and/or in combination with conventional reconcentration techniques:
1. reboiler gas space is maintained at total vapor pressure of about 25 to about 125 psia
2. reflux gas space is maintained at total vapor pressure of about 25 to about 125 psia
3. overheads from reflux pass to compressor with inlet pressure of about 25 to about 125 psia (without intermediate compression by a compressor having a lower or atmospheric inlet pressure)
4. concentration level of desiccant in lean desiccant, as compared to level in wet desiccant mixture fed to reboiler (weight percent, based on weights of water and desiccant) has been increased by at least about two percent in gas stripper that is downstream of reboiler;
5. stripper, receiving liquid from reboiler, is operated with its gas space at super-atmospheric pressure;
xe2x80x83more preferably, stripper, receiving liquid from reboiler, is operated with its gas space at total vapor pressure of about 25 to about 125 psia
6. stripper completes reconcentration at super-atmospheric pressure, contacting partially reconcentrated desiccant with at least about 5 SCF and more preferably about 6 or about 7 (standard cubic feet) per gallon of desiccant circulated, to reconcentrate the desiccant to a desiccant content of at least about 99% (weight percent, based on weights of water and desiccant)
7. reconcentration is completed at about 25 to about 125 psia without substantial removal of water at sub-atmospheric pressure
8. entire reconcentration in reboiler, and in stripper when used, is conducted at about 25 to about 125 psia without substantial removal of water at sub-atmospheric pressure
9. stripper overheads eventually pass, directly or (preferably) after mixing with reflux overheads, to compressor with inlet pressure of about 25 to about 125 psia
10. substantially all desiccant soluble VOCs and other pollutants that were present in natural gas prior to its dehydration are:
substantially all burned and thus not released as such to the atmosphere
mixed with combustible gaseous fuel to produce a mixture in which the VOCs so mixed represent less than about 10 mole percent of the total mixture, and burning the resultant mixture
mixed with combustible gaseous fuel composed predominantly of hydrocarbons other than aromatics to produce a mixture in which the aromatic content is less than about 2 mole percent of the total mixture, and burning the resultant mixture
the combustible gaseous fuel is natural gas
the combustible gaseous fuel is plant gas, gas burned for generating process heat within the plant facility in which reconcentration of the desiccant is conducted or within an adjacent facility
burned in a thermal and/or catalytic oxidation process, such as in an incinerator
burned without further compression
introduced into burner at about 25 to about 125 psia
fed, without intermediate compression, to a compressor having an inlet suction in the range of about 25 to about 125 psia, compressed in said compressor and then burned
combustion occurs after VOCs have been compressed, separated from water, returned to the contactor with additional wet gas, subjected to drying treatment in the contactor, discharged from the contactor with dried gas and transmitted via pipeline to remote industrial and domestic fuel users
11. discharging VOCs directly to a compressor used to compress
a. flash gas, and/or
b. low pressure produced gas
Other novel aspects of the invention will be apparent to those skilled in the art upon considering the teachings of the accompanying drawing and of the text which follows.
Different embodiments of the invention may possess one or more of the following advantages. Certain particularly preferred embodiments will provide all of the following advantages.
The processes of the present invention can be conducted without discharge of VOCs to the atmosphere. The invention affords an opportunity to burn substantially all desiccant-soluble VOCs and any H2S present in natural gas prior to its dehydration. VOCs and any H2S that were present in the wet desiccant may be burned without further compression.
No environmental testing of the facility for atmospheric discharge of VOCs should be required since no VOCs will be released to the atmosphere as is oftentimes the case with conventional technology. Furthermore, discharge of VOCs from the reconcentration operation at considerably higher than normal pressures facilitates mixing and extreme dilution of VOCs in plant fuel or in fuel sold and transmitted to remote users.
As compared with prior stripping practices, reconcentration of desiccants to very low water concentrations can be accomplished without wastage of stripping gas. Thus, stripping can be used to remove trace amounts of water from partially reconstituted desiccant without a large economic penalty for stripping gas utilization, even though relatively large volumes of stripping gas are required.
Due to this invention eliminating the stripping gas wastage, stripping gas units can now be used where vacuum units would have previously been considered. As compared to reconcentrators using a vacuum, excellent reconcentration of desiccants, e.g. to 99% by weight and higher, can be obtained at low enough temperatures to very satisfactorily control decomposition without maintaining a vacuum in any part of the system. Thus, it is not necessary to employ vacuum pumps, eliminating their associated capital and operating costs and the related maintenance problems. Moreover, high levels of reconcentration can be attained with TEG at reboiler temperatures of about 400xc2x0 F. and below without the use of vacuum.
When the reconcentrator has a reflux section gas space which is operated at a total vapor pressure of 25-75 psia, the water vapor, VOCs and other gaseous (including vaporous) materials present in the reflux overheads may be fed conveniently without intermediate compressors, and without their capital and operating costs, to a variety of downstream equipment. In particular, this includes equipment that will readily accept such overheads only if they are at substantially higher than normal pressure, as compared to the reflux outlet pressures of prior reconcentrators.
Examples of such receiving equipment include burners that require, for their dependable operation, that fuel be delivered to them at pressures substantially above atmospheric. Other examples include compressors installed in the plant for purposes other than compressing such reflux overheads, such as compressors required for compressing natural gas produced at low pressure, so that the gas may be charged to a dehydrating tower. These compressors may operate at an inlet pressure of 25-75 psia and thus require a like pressure in the gaseous material which is fed to them. Thus, use of the invention affords an opportunity to pass reflux overheads from the reconcentrator to such compressors without the need to provide additional step-up compressors.
Since disposition of the VOCs from the still overheads can be handled by any of the methods as described herein, the installation of equipment for the specific purpose of recovering the VOCs can be eliminated. This equipment typically includes condensers, separators, pumps, and a means for burning or incinerating the non-condensible vapors.
The larger than normal amount of stripping gas which is used in the stripping operation results in less VOCs in the condensed water vapor from the overheads since the vapor-liquid equilibrium is shifted in such a way as to minimize the condensation of the VOCs. Thus, the disposal water will contain significantly lower quantities of undesirable components. Similarly, for those systems in which the overheads are mixed with other gases, the volume ratio of the overheads to the mixing gas, such as gases for fuel and compressor suction, will be such as to achieve the same effectxe2x80x94less VOCs in the condensed water.
Other advantages will no doubt be apparent to those skilled in the art upon operating the process.